At least three separate security cameras captured the vicious incident and the hunt led to 19-year-old Stefan Sylvestre -- who they found with acid burns on his hands.

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Sylvestre eventually told authorities he'd been commissioned to throw the acid at Piper by Danny Lynch -- a man Piper had started dating just two weeks before the attack.

Lynch was a 33-year-old martial arts enthusiast who likely first saw Piper when she was working at martial arts matches as a ring girl. Piper said they initially met online.

"The first initial contact was on Facebook, where he messaged me...his profile picture was him in a martial arts outfit. And then, the next few days we carried on, talking online, and eventually swapped numbers," she said. "I really sort of quite fancied him, and the whole image he portrayed to me I really liked."

A night out led to another and the relationship moved very quickly. "One minute, you know, she's single, the next minute she's met this guy. And, you know, that's the first I heard about Danny," said Piper's best friend Kay Little.

"It was an intense relationship in that he was very affectionate, and...whenever I wasn't with him...he was calling me or texting me or e-mailing me and wanted to see me," Piper said.

But one night, Piper learned just how terrifying her new boyfriend was.

"I remember thinking about my mom and dad and all my friends, thinking I'm never gonna see them again," she recalled about the tormenting incident with Lynch. "I genuinely didn't think I would live… he was just unhinged, unstable, and extremely violent."

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After a frightening incident, Lynch made Piper promise to keep what had happened between them a secret. Scared for her life, she agreed.

Piper on Lynch: 'He Was Very Jekyll-and-Hyde'

Afraid to go to the police, Piper returned to her apartment and called Little.

"I mean, she was petrified. She was so scared and ...it was almost like she was in a whirlwind of not knowing what to do because he had been so violent and he had threatened to kill her if, if she sort of brought it up with anyone," Little said.

Traumatized, Piper stayed in her apartment for a few days, but was bombarded by Lynch with messages.

"He was very Jekyll-and-Hyde. He was so emotionally unstable," she said. "He would be, 'I love you,' and this is how strange he was, you know, 'have -- have I ruined things between us?'"

After three days of harassment, Katie finally agreed to go to a local Internet cafe to read a message Lynch had posted on her Facebook page.

"He said, 'If you just go read this last message...I promise, you'll never, ever speak to me again, and I will stop contacting you,'" Piper recalled. "And I think at this time I was kind of delirious, you know, I was broken, and… [an] opportunity somebody would've given me to get this man out of my life, I just wanted him gone."

Model's Face Melts in Vicious Attack

Still on the phone with Lynch, Piper headed out to pick up the message. He asked her to describe what she was wearing and to tell him when she walked out the front door of her building.

"He was like, 'OK, and, and what about your hair,' I said, 'It's up, I haven't showered,' you know... 'why are you asking me these strange questions now?'" she told "20/20."

At least three separate security cameras captured what happened next, but Piper remembers the fateful day all too well.

"Danny was on the phone listening to me screaming ...It's so painful and...So bad, and, and I knew straightaway that it was to do with Danny, he said he'd get me. He said he'd destroy me, and, and he'd done it," Piper said.

Police Investigation Reveals Attacker's Violent Past

Piper's doctors began a series of reconstructive surgeries, including a revolutionary skin graft on her face. Beyond a forest of medical concerns, Piper also obsessively worried about the legal proceedings against her short-lived boyfriend and his henchman who threw the acid.

An ongoing police investigation revealed that Lynch had a long history of violence. He had even served a prison sentence for throwing boiling water into a man's face.

"I don't think anger even begins to cover the word of what I felt, really," Piper's father David Piper said.

"I try not to think about him, because I can't, I can't believe he is a human being," her mother Diane Piper added.

Six months after the attack, Piper testified at Lynch's trial for her disfigurement. She faced her attacker from just a few feet away and spoke out about the terrible role his attacks had taken on her mind and body.

"She was wearing a full plastic mask all over her face...We were feeding her through a bag and a pump because she couldn't swallow solid food. ...And the judge looked at her and said, 'Are you sure you're fit enough for this?'" Piper's father recalled. "And I looked at her and she looked pathetic. And I thought, 'After all you've been through, fighting for your life, now you gotta go through this.'"

A jury found Stefan Sylvestre, the man who threw the acid, guilty of causing bodily harm. He received a life sentence, with a minimum of 12 years in prison. Lynch received two life sentences, and will serve at least 16 years in jail.

With Attacker Behind Bars, Piper Rebuilds Her Life

Knowing that Lynch would be behind bars was an important milestone for Piper.

"She was very scared that he would come back and hurt her again," said Dr. Mohammad Jawad, who spearheaded Piper's treatment. "So when that thing happened, and, and they had given [the] sentence, I could see a huge change in Katie's world."

"I feel safer," she said. "Unfortunately, the life sentence I have imposed on me, really is till the day I die."

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